Method of measuring insulation resistance of capacitor and apparatus for screening characteristics

ABSTRACT

The insulation resistance of a capacitor is accurately measured within a short period of time by applying AC signals at two different frequencies f1 and f2 to the capacitor to measure the impedance Z1 and Z2 of the capacitor at each frequency. The series resistance Rs and capacity C of the capacitor are obtained from the impedance Z1 at the higher frequency f1, and the insulation resistance Rp of the capacitor is obtained from the impedance Z2, series resistance Rs and capacity C at the lower frequency f2.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of measuring the insulationresistance of a capacitor and an apparatus for screeningcharacteristics.

2. Description of the Related Art

In general, a capacitor is determined as good or defective by measuringthe insulation resistance of the same. When a capacitor is not charged,the insulation resistance thereof can not be measured correctly becauseof the capacity of the capacitor. Under such circumstances, a method iscommonly used wherein a DC voltage is first applied to the capacitor toprecharge it and a leakage current (charging current) is measuredthereafter to measure the insulation resistance of the capacitor.Obviously, a good part has a low leakage current.

Known conventional methods for measuring insulation resistance includethe method of measurement defined in Japanese Industrial StandardJIS-C5102. However, since a current value must be measured on acapacitor which has been sufficiently charged according to this method,a charging time as long as approximately 60 sec. has been required. Theneed for electronic apparatuses at reduced cost and with improvedreliability has resulted in a need for improvement on the productioncapacity and quality of electronic components such as capacitors. It istotally impossible to satisfy such a need using the conventional methodof measurement that requires such a long charging time per capacitor.

It is therefore an object of the present invention to provide a methodof measuring the insulation resistance of a capacitor wherein theinsulation resistance of a capacitor can be quickly and accuratelymeasured.

It is another object of the present invention to provide an apparatusfor screening the characteristics of capacitors wherein acharacteristics measuring step and a packing step can be linked toimprove operational efficiency and to reduce the size and cost of thefacility.

SUMMARY OF THE INVENTION

In order to achieve the above-described objects, according to a firstaspect of the invention, there is provided the steps of: applying ACsignals at two different frequencies to a capacitor to measure theimpedance of the capacitor at each of the frequencies the frequency f₁being higher than the frequency f₂; obtaining series resistance Rs andcapacity C of the capacitor from the impedance at the higher frequency;and obtaining insulation resistance Rp of the capacitor from the seriesresistance Rs, the capacity C and the impedance at the lower frequency.

The measurement of insulation resistance according to the prior artwherein a DC current is applied for a long time has encountered aphenomenon that the apparent capacitance is reduced depending on thetype of the capacitor. For this reason, the prior art involves a thermalprocess referred to as “thermal recovery” performed on a good capacitoron which the measurement of insulation resistance has been completed torecover the initial capacitance. This has resulted in an additionalprocess following the measurement and has reduced operational efficiencyfurther. On the contrary, according to the present invention, since itis only required to apply AC signals for a very short period, thecapacity of a capacitor is not reduced and the need for “thermalrecovery” process is therefore eliminated.

Further, a conventional apparatus for screening characteristics employsa large turn table having retaining portions to retain capacitorsbecause it requires a measuring time of about 60 seconds per capacitorand requires operations such as establishing a charging area thatsurrounds a major part of the turn table and stopping the turn table fora predetermined time for charging, which has resulted in very lowoperational efficiency. In addition, it is necessary to keep capacitorsdetermined as good in an unloading container temporarily and to pick upthe capacitors from the unloading container one by one using a partsfeeder or the like to supply them to a taping device or the like. Thishas significantly slowed operations from the measurement ofcharacteristics up to packing and has resulted in increases in the sizeand cost of the facility.

On the contrary, when an apparatus for screening characteristics isconfigured using the above-described method for measuring insulationresistance according to an aspect of the invention, processes from themeasurement of characteristics up to packing can be linked to each otherto improve operational efficiency and to reduce the size of thefacility. Specifically, a capacitor supplied to a retaining portion of atransporting means by a supply means is transported to an impedancemeasuring portion as the transporting means is driven to measure twokinds of impedance. The insulation resistance is then calculated by agood/defective determination means based on the impedance. While themeasurement of insulation resistance has required a precharging time aslong as 60 seconds in the prior art, the present invention makes itpossible to perform measurement within a very short period on the orderof several tens of milli-seconds because insulation resistance iscalculated from impedance at two frequencies. Capacitors are determinedas to whether they are good or defective based on measured insulationresistance; capacitors determined as defective are ejected from adefective parts ejecting portion; and capacitors determined as good aresupplied from good parts unloading portion to a packing means whereinthey can be immediately packed in a tape, case or the like.

The transporting means may be a turn table having retaining portions forretaining capacitors provided on the circumference thereof at equalpitches or an endless belt having retaining portions for retainingcapacitors provided at equal pitches.

The determination of good and defective capacitors is preferably carriedout depending not only upon the insulation resistance but also upon theelectrostatic capacity thereof. In this case, the good/defectivedetermination means may determine good and defective capacitors frommeasured values of both the insulation resistance and electrostaticcapacity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram showing the principle of a method ofmeasuring insulation resistance of a capacitor according to the presentinvention;

FIG. 2 shows the results of an experiment carried out using the methodof the present invention;

FIG. 3 is a schematic plan view of an embodiment of an apparatus forscreening characteristics according to the present invention;

FIG. 4 is a perspective view of a turn table;

FIG. 5 is a sectional view of the impedance measuring portion of theapparatus for screening characteristics in FIG. 3;

FIG. 6 is a sectional view of the good parts unloading portion of theapparatus for screening characteristics in FIG. 3;

FIG. 7 is a sectional view of the taping device of the apparatus forscreening characteristics in FIG. 3;

FIG. 8 is a schematic plan view of a second embodiment of an apparatusfor screening characteristics according to the present invention;

FIG. 9 is a schematic plan view of a third embodiment of an apparatusfor screening characteristics according to the present invention;

FIG. 10 is a schematic side view of the third embodiment of an apparatusfor screening characteristics according to the present invention;

FIG. 11 is a perspective view of an example of the belt used in theapparatus for screening characteristics in FIG. 10; and

FIG. 12 is a perspective view of another example of the belt used in theapparatus for screening characteristics in FIG. 10.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring now to the drawings and, in particular to FIG. 1, theequivalent circuit of a capacitor 1 includes a series resistance Rs, aninsulation resistance Rp and an electrostatic capacitance C. When thefrequency of an AC signal applied to the capacitor 1 from a measuringapparatus 2 is high, the impedance of C is very small compared to Rpand, as a result, a major part of the signal passes through C and a verylittle part of the signal passes through Rp. Conversely, if the signalis at a low frequency, the impedance of C becomes high to increase thepercentage of the signal that passes through Rp. Thus, the higher thefrequency of the measurement signal, the smaller the percentage of thesignal that passes through Rp. Conversely, the lower the frequency ofthe measurement signal, the greater the percentage of the signal thatpasses through Rp. The percentage of the signal that passes through Rpdepends also on the magnitude of C and Rp.

When the impedance of a capacitor 1 having a certain capacitance C ismeasured, the effect of Rp can be ignored if the measurement is carriedout using a high frequency which sufficiently reduces the percentage ofthe signal that passes through Rp, and a resultant measured value willgive Rs and C. Next, Rp can be obtained by measuring the impedance ofthe same capacitor using a low frequency at which the percentage of thesignal passing through Rp is increased to a measurable level and byeliminating the effect of Rs and C obtained above from a resultantmeasured value by means of computation. A preferable result can beobtained by shifting the two frequencies used from each other as much aspossible under the conditions for measurement and in the measuringapparatus.

The method of measuring insulation resistance according to the presentinvention will now be specifically described.

Impedance Z of the capacitor shown in FIG. 1 is expressed by thefollowing Equation. $\begin{matrix}{Z = {{Rs} + \frac{Rp}{1 + {\omega^{2}{Rp}^{2}C^{2}}} - {j\frac{\omega \quad {Rp}^{2}C}{1 + {\omega^{2}{Rp}^{2}C^{2}}}}}} & \left( {{Equation}\quad 1} \right)\end{matrix}$

where ω represents an angular frequency.

Since insulation resistance Rp is normally on the order of severalhundred MΩ or more, 1/Rp<<ωC is satisfied at a frequency as high as, forexample, about several MHz or more. Therefore, Equation 1 can be changedas follows.

Z=Rs−j(1/ωC)  (Equation 2)

Let us assume here that impedance Z₁ of a capacitor is as expressed bythe following equation when measured at a high frequency f₁ on the orderof several MHz.

Z₁=¦Z₁ ¦e ^(−jω1) =x ₁ +jy ₁ (where ω₁=2πf ₁)  (Equation 3)

Then, the following equation can be derived from Equations 2 and 3 toobtain Rs and C.

Rs=x ₁, C=−1/(ω₁ ·y ₁)  (Equation 4)

Next, let us assume that an impedance Z₂ of the same capacitor is asexpressed by the following equation when measured at a frequency f₂lower than the frequency f₁.

Z₂=¦Z₂ ¦e ^(−jω2) =x ₂ +jy ₂ (where ω₂=2πf ₂)  (Equation 5)

Then, since 1/Rp<<ω₂C is not satisfied because the frequency f₂ is low,Equation 1 is used.

Then, a parameter ξ is given as follows.

ξ=(x ₂−Rs)ω₂ ²C²  (Equation 6)

Then, the real parts of Equations 1 and 5 give the relationship shownbelow. $\begin{matrix}{{Rp} = \frac{1 + \sqrt{1 - {4{\xi \left( {x_{2} - {Rs}} \right)}}}}{2\xi}} & \left( {{Equation}\quad 7} \right)\end{matrix}$

Rp can be obtained from the above equation.

The imaginary parts of Equations 1 and 5 give the relationship shownbelow. $\begin{matrix}{{Rp} = \sqrt{\frac{- y_{2}}{\omega_{2}{C\left( {1 + {y_{2}\omega_{2}C}} \right)}}}} & \left( {{Equation}\quad 8} \right)\end{matrix}$

Rp can be obtained also from the above equation.

The insulation resistance Rp of interest can be obtained as describedabove. According to the method of the present invention, the insulationresistance of one capacitor can be obtained in about several tens ofmsec., which provides a significant improvement of operationalefficiency over the prior art.

As the apparatus for measuring the impedance (effective value and phase)of a capacitor, well-known measuring apparatuses (e.g., HP4284Amanufactured by Hewlett-Packard Company) may be used.

The first cycle of impedance measurement using the higher frequency f₁and the second cycle of impedance measurement using the lower frequencyf₂ are preferably carried out continuously with electrodes 1 a and 2 bof the capacitor 1 and measuring terminals 2 a and 2 b of the measuringapparatus 2 (see FIG. 1) kept in contact with each other. Thiseliminates any variation in contact resistance between the electrodes 1a, 1 b and the measuring terminals 2 a, 2 b and the like to allowaccurate measurement.

The higher frequency f₁ of the AC signals at two frequencies ispreferably at a value which can be ignored in the sense of its effect onthe impedance Z₁ of the insulation resistance Rp, and the lowerfrequency f₂ is preferably at a value which can affect the impedance Z₂of the insulation resistance Rp. Therefore, they are preferably set suchthat, for example, relationships “f₁≧1 MHz” and “f₂≦f₁/100” aresatisfied.

In general, it is difficult to detect contact between a capacitor andmeasuring terminals because a capacitor has very high insulationresistance. That is, it is difficult to distinguish a value measured inan open state from a value measured in a normal state. Under suchcircumstances, a special circuit has been used in the prior art todetect contact between a capacitor and measuring terminals (for example,see Japanese unexamined patent publication No. H6-130101). On thecontrary, the present invention makes it possible to easily obtain acapacitance C from impedance at the time of application of an AC signalat a high frequency f₁. Since the fact that the capacitance C can bemeasured means that there is preferable contact between the electrodesand the measuring terminals, the measurement of the capacitance C(detection of contact) and the measurement of insulation resistance Rpcan be carried out simultaneously.

FIG. 2 shows the results of an experiment wherein the electrostaticcapacity C, series resistance Rs and insulation resistance Rp wereobtained for three kinds of capacitors using the measuring circuit shownin FIG. 1.

The capacitors (types 1 through 3) were set at the following conditions.

C(pF) Rs(Ω) Rp(Ω) Type 1 31 200  100M Type 2 31 200  500M Type 3 31 2001000M

A frequency of 1 MHz was used as the measuring frequency f₁, whereas twofrequencies 1 kHz and 133 Hz were used as the measuring frequency f₂.Rs, C and Rp were obtained based on measured impedance using Equations 4and 7. As apparent from FIG. 2, the results of the calculation are at ahigh level of correspondence to actual values, which proveseffectiveness of the method of the present invention.

FIG. 3 shows a first embodiment of an apparatus for screeningcharacteristics utilizing the method of the present invention.

Reference numeral 10 designates a turn table as an example of thetransporting means, and the turn table 10 is intermittently driven forrotation at constant pitches in the direction of the arrow. As shown inFIG. 4, the turn table 10 has concave retaining portions 11 each capableof retaining one chip type capacitor 1 provided on the outercircumference thereof at the same pitches as the driving pitchesdescribed above. At spaced intervals about the turn table 10 there are asupply portion 12 for supplying capacitors 1 to the turn table 10, animpedance measuring portion 13 for measuring impedance, a defectiveparts ejecting portion 14, and a good parts unloading portion 15. Asupply device 16 such as a parts feeder is provided in a positionassociated with the supply portion 12 for feeding the capacitors 1 tothe turn table 10 one by one. Further, the impedance measuring portion13 includes an impedance measuring device 2 (see FIG. 1) having a pairof measuring terminals 2 a and 2 b which contact electrodes 1 a and 2 bof the capacitors 1. Two kinds of measured values Z₁ and Z₂ obtained bythe measuring device 2 are sent to a good/defective determinationportion 17 where Equations 4 and 7 are calculated to obtain values ofRs, C and Rp and to determine whether the capacitors 1 are good ordefective.

Capacitors 1 determined as defective are ejected to the outside at thedefective parts ejecting portion 14, and capacitors determined as goodare unloaded at the good parts unloading portion 15 to a base tape 31.

As shown in FIG. 5, the turn table 10 is formed with an air vent 18associated with each of the retaining portions 11, and each of the airvents 18 is connected to a positive pressure source 19 and a negativepressure source 20 through an electromagnetic change-over valve 21. Thechange-over valve 21 selectively connects the positive pressure source19 or negative pressure source 20 to the air vent 18 based on a commandsignal from the good/defective determination portion 17. While acapacitor 1 is contained in a retaining portion 11, the air vent 18 isconnected to the negative pressure source 20 to retain the capacitor 1against the inner circumferential surface of the retaining portion 11 bymeans of suction. As a result, the measuring terminals 2 a and 2 bcontact the electrodes 1 a and 2 b of the capacitor 1 at constantpositions to allow stable measurement of characteristics and to preventthe capacitor 1 from coming out due to a centrifugal force produced bythe rotation of the turn table 10. When a defective capacitor 1 reachesthe defective parts ejecting portion 14, the change-over valve 21 isswitched Gil to the position of the positive pressure source 19 to blowair to eject the capacitor 1. Similarly, when a good capacitor 1 reachesthe good parts unloading portion 15, the change-over switch 21 isswitched to the position of the positive pressure source 19 to push thecapacitor 1 contained in the retaining portion 11 on to the base tape 31in which the capacitor is contained in a recess 31 a thereof as shown inFIG. 6.

A taping device 30 is provided in a position associated with the goodparts unloading portion 15 to supply the base tape 31 to the turn table10 in the tangential direction thereof such that the tape is atsubstantially the same height as the retaining portion 11. As shown inFIG. 7, the taping device 30 is comprised of a supply roll 32 forsupplying the base tape 31 having the recesses 31 a for containingcapacitors, a guide roller 33 for guiding the base tape 31, a supplyroll 35 for supplying a cover tape 34, a pressure roll 36 for urging thecover tape 34 against the base tape 31 to bond it thereon, a guideroller 37 for guiding the tapes 31 and 34 bonded together, a take-uproller 38 for taking up the tapes 31 and 34 bonded together, and thelike. The take-up roller 38 is intermittently driven by a driving means(not shown) for stepping on a pitch-by-pitch basis in the direction ofthe arrow. The timing for this driving is in synchronism with the timingat which the turn table 10 is driven. Thus, when a retaining portion 11of the turn table 10 is stopped at the good parts unloading portion 15,the base tape 31 is also stopped at the good parts unloading portion 15at the same time. Then, air is blown from the air vent 18 provided onthe turn table 10 to push out the capacitor 1 contained in the retainingportion 11 on to the base tape 31 on which it is contained in the recess31 a thereof. After the capacitor 1 is placed in a recess 31 a, thecover tape 34 is bonded on to the base tape 31 to seal the recess 31 a.

When a defective capacitor 1 is ejected at the defective parts ejectingportion 14, the retaining portion 11 becomes vacant. Therefore, thecover tape 34 may sometimes be bonded to the base tape 31 with therelevant recess 31 a left vacant if the turn table 10 and the tapingdevice 30 are always in synchronism with each other. In order to solvethis problem, a sensor (not shown) is provided immediately before thegood parts unloading portion 15 to detect the presence and absence of acapacitor 1 in a retaining portion 11. When this sensor detects theabsence of a capacitor 1 in a retaining portion 11, the taping device 30is temporarily stopped to allow the turn table 10 to pass therethrough,so that every recess 31 a on the base tape 31 can contain one capacitor1.

FIG. 8 shows a second embodiment of an apparatus for screeningcharacteristics.

In the second embodiment, supply devices 41 such as parts feeders areprovided on both sides of one turn table 40, and two taping devices 42are provided. Reference numeral 43 designates supply portions, 44designates impedance measuring portions; 45 designates defective partsejecting portions; and 46 designates good parts unloading portions. Inthe present embodiment, two tapes 42 a are transported in oppositedirections.

This apparatus is characterized in that although the turn table 40 islarger than the turn table 1 in FIG. 3, the two taping devices 42 can beprovided for one turn table 40 to allow further improvement ofoperational speed and efficiency compared to the case shown in FIG. 3.

FIG. 9 shows a third embodiment of an apparatus for screeningcharacteristics.

In the third embodiment, a case packing device 50 is used as the packingdevice. Good capacitors that have been unloaded from a parts feeder 51through a turn table 52 are packed in a case 53 by the case packingdevice 50. The case 53 can contain a predetermined number of capacitorsand is driven by one pitch in the direction of the arrow after thepredetermined number of capacitors are packed. Reference numeral 54designates a supply portion; 55 designates an impedance measuringportion; 56 designates a defective parts ejecting portion; and 57designates a good parts unloading portion.

This embodiment has the same effect as that illustrated in FIG. 3. Thisembodiment may employ two or more case packing devices 50 for one turntable 52 as that shown in FIG. 8.

FIG. 10 shows a fourth embodiment of an apparatus for screeningcharacteristics.

In the fourth embodiment, an endless belt 60 is used as the transportingdevice instead of a turn table. The belt 60 is intermittently orcontinuously driven by a drive pulley 61 and a guide pulley 62 in thedirection of the arrow. A parts feeder 63 is provided at the startingside of the belt 60 and is followed by an impedance measuring portion64, a defective parts ejecting portion 65 and a good parts unloadingportion 66 in the order listed. Air nozzles, ejection pins or the likeare provided at the defective parts ejecting portion 65 and good partsunloading portion 66 to remove capacitors 1 from the belt 60.

Referring to the specific structure of the belt 60, for example, aninsulated holder 71 made of resin, rubber or the like may be mounted ona steel belt 70, and one capacitor 1 may be contained in each ofretaining holes 72 formed on the upper surface of the holder 71, asshown in FIG. 11. In this case, the steel belt 70 is formed with feedholes 73 on both sides thereof at constant pitches, and the feed holes73 may be engaged with protrusions provided on circumferential surfacesof the pulleys 61 and 62 to achieve highly accurate feeding.

The belt 60 may have another structure wherein recesses 81 are formed onthe outer circumferential surface of a rubber belt 80 at constantpitches and wherein one capacitor 1 is contained in each of the recesses81, as shown in FIG. 12. Guide plates 82 and 83 are provided on bothsides of the rubber belt 80 to slidably guide the rubber belt 80 and toprevent capacitors 1 from dropping. Inner teeth 84 may be formed on theinner circumferential surface of the rubber belt 80 and engaged withouter teeth formed on the circumferential surfaces of the pulleys 61 and62 to achieve highly accurate feeding.

The present invention is not limited to ceramic capacitors and may beapplied to any capacitors including electrolytic capacitors and filmcapacitors.

The transporting means is not limited to a turn table or belt and othertransporting means may be used. The method of measuring impedance is notlimited to the method wherein a capacitor is measured by urgingmeasuring terminals against it from above while it is held on atransporting means. Alternatively, measuring electrodes may be formed ona transporting means in advance, and a capacitor may be put inconduction to the measuring electrodes by urging the capacitor fromabove using an insulator.

The method of driving a transporting means is not limited to theintermittent method of driving, and continuous driving may be employed.

Although the determination of good and defective parts is made based onthe results of measurement of both capacity and insulation resistance inthe above-described embodiments, the determination of good and defectiveparts may be carried out independently based on each of capacity andinsulation resistance.

As apparent from the above description, according to the presentinvention, the impedance of a capacitor is measured using two kinds ofAC signals and the insulation resistance is measured based on theimpedance. This allows the measurement of insulation resistance to beperformed within a very short period of time. As a result, speed of ameasuring operation can be significantly increased.

Further, an apparatus for screening characteristics according to thepresent invention can obtain the insulation resistance of a capacitor ina short period of time. Therefore, a capacitor on which characteristicsmeasurement has already been completed can be directly transferred froma transporting means to a packing means such as a taping device. Thisallows operations from the measurement of characteristics up to packingto be fully automated and therefore makes it possible to improve thespeed of operation significantly and to reduce the size and cost offacility compared to the prior art.

It should be understood that the foregoing description is onlyillustrative of the invention. Various alternatives and modificationscan be devised by those skilled in the art without departing from theinvention. Accordingly, the present invention is intended to embrace allsuch alternatives, modifications and variances which fall within thescope of the appended claims.

What is claimed is:
 1. A method of measuring insulation resistance of a capacitor, comprising the steps of: applying AC signals at two different frequencies f₁ and f₂ to a capacitor to measure impedance Z₁ and Z₂ of the capacitor at each of the frequencies, the frequency f₁ being substantially higher than the frequency f₂; obtaining series resistance Rs and capacitance C of the capacitor from the impedance Z₁ at the higher frequency f₁; and obtaining insulation resistance Rp of the capacitor from the series resistance the capacitance C and the impedance Z₂ at the lower frequency f₂, wherein the frequencies f₁ and f₂ of said AC signals satisfy two conditional expressions: f₁≧1 MHZ; and f₂≦f₁/100. 